[yaml2obj] - Map section names to chunks for each ELFYAML::ProgramHeader early. NFCI.

Each `ELFYAML::ProgramHeader` currently contains a list of section names
included. We are trying to map them to Fill/Sections very late,
though we can create such mapping early, in `initProgramHeaders`.

The benefit is that with such change it is possible to access mapped
chunks earlier (for example during writing section content) and have
simpler code.

Differential revision: https://reviews.llvm.org/D80520